The field of the present invention relates to imaging of bodies, examples being ultrasonic, nuclear magnetic resonance, and x-ray imaging which are used commonly as diagnostic techniques for medicine, non-destructive testing, and quality control in industry.
In 1933, Debye predicted that passage of an ultrasonic wave through a solution of electrolytes would result in the generation of a voltage. The potential generated has become know as the ultrasonic vibration potential. After experimental confirmation of the effect, it was found that colloidal suspensions produced a large signal. The effect is thus also known as the colloidal vibration potential.
The principle of the effect, as well as experimental and theoretical findings have been reviewed by Zana and Yeager, by Povey in his text, and O""Brien, Cannon, and Rowlands. The theory of the electroacoustic effect can be found in these reviews, the references therein, and in the early paper by Hermans. O""Brien and coworkers over the years have developed a detailed theory of the effect; much of his theory can be found in the papers disclosed.
The electroacoustic effect takes place when an ultrasonic wave passes through a fluid containing electrolytes or colloids. In the case of an electrolyte, the different inertias of the ionic species in an electrolyte solution cause them to move to a greater or lesser extent in response to the fluid motion that constitutes a sound wave. The microscopic charge separation that follows from their different dynamic response, when added over the interaction region of the sound wave, results in the macroscopic voltage. Further details can be found in the original paper of Debye. Voltages are also produced when a sound wave passes through a colloidal suspension. Consider the usual case of a colloidal suspension of particles in a fluid where the particles have a higher density than that of the surrounding fluid. Here, the higher mass of a particle relative to that of the fluid volume it displaces means that when an ultrasonic wave passes, the particle motion does not exactly follow the fluid motion, but rather lags it with both a smaller displacement in space and velocity. The different motions of the fluid and particles are described by the equations of fluid dynamics and follows as a result of the higher inertia of a dense particle relative to that of an equivalent volume of the fluid. Colloidal particles are charged bodies with a so-called xe2x80x9czetaxe2x80x9d potential, surrounded by a cloud of the opposite charge. The solution thus has overall charge neutrality. The presence of the ultrasonic wave gives rise to a charge separation that arises from distortion of the charge cloud around the particle when the particle fails to move in phase with the fluid. When an ultrasonic wave passes, the fluid carries along the counter charge but the particle and its charge remains more stationary in space. The result of the different motions of the particle and surrounding fluid is that a dipole is generated at the site of each particle, which, when added over a half cycle of an acoustic wave (where the velocity is unidirectional) adds to give a macroscopic voltage, the frequency of the voltage being governed by the frequency of the ultrasonic acoustic wave.
The magnitude of the vibration potential generated in a colloidal suspension, as given by O""Brien, or as summarized in the literature from Matec, Inc., is proportional to the density difference between the particle and the fluid, the volume fraction of the particles, the dynamic mobility, the inverse of the conductivity, and the magnitude of the ultrasonic velocity. Zana and Yeager give somewhat different expressions that involve relaxational parameters, the thickness of the ionic atmosphere, the number density of particles, the particle charge, the dielectric constant, and solvation volume. The magnitude of the signal produced as the ultrasonic wave traverses the body depends on the above quantities; however, the exact details of the theory are not important for the operation of the imaging device described here. Note that in the case of particles with densities lower than that of the fluid, the motion of the particle is opposite to that described above. Again, a voltage is produced, but, for the same relative charge of the particle and fluid, the opposite polarity dipole and overall voltage are produced on each acoustic cycle.
Practical application of the ultrasonic vibration potential to characterization of suspensions has been reported. Freeman describes a device useful in industrial chemical processes for detecting particles or the change in concentration of particles during a chemical process based on the ultrasonic vibration potential. Likewise, Oja, Petersen and Cannon describe a device for characterizing the bulk properties of particulate suspensions using the vibration potential. The Matec Inc. sales literature describes a commercially available device for fluid characterization.
It is to be noted parenthetically that there is another electroacoustic effect that is essentially the reverse of that described above, whereby a voltage is applied to a fluid and an ultrasonic wave is produced. The acoustic wave magnitude is referred to as the xe2x80x9celectrokinetic sonic amplitudexe2x80x9d, as described in the brochure by Matec, Inc. cited above. It is possible to determine colloidal properties of a bulk sample placed in a cell with the device manufactures by Matec that measures the characteristics of sound wave produced following application of a voltage to the cell.
Description of the principle of imaging through the ultrasonic vibration potential has been given by Diebold and Beveridge. An image formed using the ultrasonic vibration potential is thus a map of the response of electroacoustic signal in space, which, in turn, is dependent on the quantities that appear in the theories of the effect as given above. Mixtures of electrolytes and colloidal particles would have a response that is a combination of the responses of the electrolytes and colloidal particles. Whether the body contains electrolytes or colloids alone, or has mixtures, for a given ultrasonic wave of a given frequency and amplitude, there is some voltage response to the wave in fluid. The voltage produced has a magnitude that depends on the ultrasonic wave and the fluid properties. Hereinafter the word colloid will be used for colloids, electrolytes, or mixtures of electrolytes and colloids.
The present document summarizes some of the theoretical aspects of the problem given in the paper by Diebold and Beveridge, and adds new ideas concerning principles and the operation of an actual device. It is to be noted that although the invention uses ultrasonic waves, it is fundamentally different than the well-known method of ultrasonic imaging since the latter records a reflected wave as the means of imaging while the invention described here records a voltage produced by the object itself in response to the ultrasonic waves.
It is therefore an object of the present invention to provide a means for formation of an image of a body. Imaging is carried out for the same diagnostic purposes as in x-ray imaging, NMR imaging, ultrasound imaging, or photoacoustic imaging, namely, for visualizing the inside of bodies. Such information is useful for diagnostic purposes, as in medicine and nondestructive testing.
It is also an object of the present invention to provide a means of image formation based on a different principle, namely, the electroacoustic effect, which will have properties unique to the method, in particular contrast, based on a completely new principle for imaging, namely, the ultrasonic vibration potential.
Consider a one-dimensional response, which in the case of a colloidal suspension, would be a concentration of the suspension that varies as a function of distance from the launching point of an ultrasonic wave. Take a 10 cm thick layer with the launching point of the ultrasonic wave at x=0, and with non-colloidal xe2x80x9cinertxe2x80x9d fluid extending throughout the 10 cm region except between points 3 and 5 cm from the origin where a colloidal suspension is found. The region between 3 and 5 cm will be referred to as the active region. Electrodes are placed at the points x=0 and x=10 cm where the voltage is recorded. It is assumed that the body is a weak conductor of electricity and that the voltage generated at its endpoints can be sensed with a high input impedance amplifier. The pulsed ultrasonic wave in this example is considered as having a wavelength much smaller than 1 cm; say 0.1 mm, and a pulse width of a duration that corresponds to 1 mm, so that about ten cycles are in the pulse. On launching the ultrasonic wave no signal is produced until the first half cycle of the wave reaches the 3 cm point, enters the colloid, and produces a uni-polar voltage. When the second half cycle of the wave reaches the interface, it begins to generate a voltage of the opposite polarity, subtracting from that generated by the first part of the wave. The process repeats itself as subsequent cycles of the ultrasonic wave enter the active region containing the colloid giving an alternating voltage at the electrodes. Further, for the simple case presented here, it can be seen that the output voltage is proportional to the integral of the acoustic velocity in the pulse from the point x=3 to the point x=5. When the pulse is totally inside the active region, the integral of the pulse velocity is zero and no voltage is produced between the points x=3 and 5, and of course, x=0 and 10 where the electrodes are placed. Next, the pulse begins to exit the active region. As the pulse reaches x=5 cm, the part of the pulse immediately extending outside the active region gives no signal, but the part remaining in the active region produces a voltage. The result is that an overall voltage is again recorded with the electrodes. An alternating voltage is produced again as the pulse moves in space until all of the acoustic pulse exits the active region. When the pulse is wholly in the region x greater than 5, no voltage is produced. When the magnitude of the ac voltage signal versus time data are plotted as Voltage (magnitude) versus Z (distance), a plot such as shown in FIG. 2 would be generated. The active region between Z=3 cm and Z=5 cm, where the vibration potential is generated, can be seen on the plot as defined by the peaks at these two points.
It can be seen that a recording of the voltage gives a profile of the colloid spatial distribution within the body, in this case the size of the ac voltage indicating that the pulse is entering or leaving the active region containing colloid. Knowledge of the sound speed permits transformation of the voltage vs. time profile into a colloid xe2x80x9cresponsexe2x80x9d vs. distance profile. Of course response involves all of the factors given by theory. Such a profile can locate objects within a body. When sharp boundaries are not present between the colloid and its inert surroundings, description of the voltage production must include the concept of a xe2x80x9cresponse gradientxe2x80x9d, which would describe the gradient of the colloid concentration in space. The details of the voltage generation process can be described as proportional to the integral of the response gradient of the medium with the ultrasonic pulse. Note that plots of signal vs. distance in the sample can be given meaning through empirical methods by correlating the features of plots with known features in control samples. A detailed knowledge of the theory of signal production is not essential for obtaining useful information.
Simple examples of the use of the device would be for determining the location of blood or blood vessels within the human body, or the presence of colloidal fluids in opaque, weakly conducting objects. Chodorow has shown that blood can be detected through use of the ultrasonic vibration potential. In the case of blood veins, for instance, the voltage response of the electroacoustic imaging device would be greater for the blood vein than for muscle or fatty tissue since the blood contains both electrolytes and cells that are colloidal in nature. Thus, a large contrast in the image would be expected at the site of blood or lymph in proximity to tissue with a low fluid content. A possible application of the imaging method would be in detecting the dimensions of arteries and veins, and the early detection of tumors, which are known to have increased vascularization. With regard to living tissue, the device would be to a certain extent a blood detector. From the standpoint of nondestructive testing, the presence of a fluid in a within solid or porous material would be detectable since the solid cannot produce a substantial electroacoustic effect, but the fluid can.